delta15N Studies of Nitrogen Use by the Red Mangrove, Rhizophora mangle L. in South Florida

To help define nitrogen (N) sources and patterns of N processing in mangrove ecosystems, mangrove leaf nitrogen contents and delta15N values were assayed in three marshes along the south Florida coast. In each marsh, leaf samples were collected from dwarf mangroves at interior locations and taller mangroves at the ocean fringe. Leaf % N and delta15N values did not differ consistently between dwarf and tall mangroves, even though there were large variations in mille15N (18mille range, -5 to +13mille) and % N (1·2% range, 0·9-2· 1%). Highest % N and delta15N values occurred along the western margin of Biscayne Bay where canals draining agricultural lands deliver high-nitrate waters to fringing mangrove marshes. High mangrove delta15N values may be good biomonitors of anthropogenic N loading to south Florida estuaries. Lower values likely reflect less anthropogenic N entering the mangrove marshes, as well as differences in plant physiology that occur along the fringedwarf gradient.